In the field of optical fiber filters, it is known to make Bragg gratings in core sections of such optical fibers. Bragg gratings are made by periodically changing refractive index in the fiber material. Such changes are obtained by irradiating said sections of the fiber core with ultraviolet radiation. The change in refractive index caused by exposure to light is known as the "photo-refractive effect". This effect is permanent. The property of a material having an index capable of being modified under such light irradiation is referred to herein as "photo-sensitivity". In present technology, photo-sensitivity characteristics are related to a defect in germanium present in the silica matrix of the optical fiber. Other dopants making the core of the fiber photo-sensitive can also be used. The advantage of germanium is that it is normally present in the core of an optical fiber since it serves to increase the refractive index of the core of a fiber relative to the index of the cladding around the core. This index increase, also known as an "index step", serves to guide light signals in the core of the fiber.
During manufacture of an optical fiber, different layers of doped and undoped silica are deposited in succession inside a tube, with the layers adhering to the inside wall of the tube to build up progressively the various layers that are to constitute the optical fiber. The diameter of a preform made in this way is greater than the diameter of the fiber. The fiber is subsequently obtained by collapsing and drawing down the preform while hot.
To make a Bragg grating, a section of the fiber core that is to act as a filter is subjected selectively to periodic ultraviolet irradiation. This irradiation gives rise to permanent local changes of refractive index. These changes are linked to chemical and structural modification of the bonds of germanium atoms in the core. The variation in the value of the refractive index in the fiber core that results from these changes can be as great as a few parts per thousand.
The grating is then in the form of modulation in the refractive index along the section forming an attenuating fiber.
Conventionally, when the changes of the index grating are perpendicular to the axis of the optical fiber, the quantity of light that is not transmitted by the fiber is reflected back along the core of the optical fiber with reflection being at a maximum at the Bragg wavelength determined by a resonance condition. Physically, coupling is created between the fundamental mode propagating forwards and the mode propagating in the opposite direction.
Depending on the length of the section that has been subjected to exposure, on the period at which changes repeat along said section, and on the strong or weak nature of the changes (depending on the large or small variation in refractive index where the changes occur), it is possible to modify the following transmission characteristics respectively: bandwidth, center frequency, and degree of attenuation.
When the photo-induced index variations are strong, there also occurs coupling of the fundamental mode in the cladding modes at shorter wavelengths. According to the article "Optical fiber design for strong gratings photo-imprinting with radiation mode suppression" given at the OFC Conference San Diego 95, Post Deadline 5, by E. Delevaque et al., this can be avoided by doping a portion of the cladding that is close to the core with germanium. A fluorine co-dopant is then added in the cladding so as to re-establish the index step.
In a particular application, attempts have been made with such filters to compensate for the non-flat gain characteristic of the amplifiers used along very long distance optical links. Over very long distances, particularly those passing via undersea cables, the per kilometer attenuation of waves in the optical fibers is such that it is necessary to install optical amplifiers at intervals. It is known that such amplifiers unfortunately present the drawback of systematically favoring certain frequency components within the transmitted band.
This phenomenon becomes particularly troublesome when such optical amplifiers are used in wavelength division multiplex (WDM) type links, where different channels are conveyed by optical carriers at different frequencies for the purpose of increasing the overall capacity and the modularity of the system. Given the phenomenon that takes place within the optical amplifier, such favoring of particular wavelengths would be completely unaccepted if it were not regularly compensated. In this application, the aim is specifically to flatten the gain of erbium-doped optical fiber amplifiers. Naturally other applications could be envisaged.
This type of Bragg grating filter thus suffers from the drawback of acting as a partial reflector of those components of the amplified signal that are concerned by the filtering. A portion of the optical signal at these frequencies is thus returned by reflection into the optical amplifier. As a result, in the amplifying section, not only does the signal reflected by the filter return and interfere, but also the signal back-scattered by the fiber is returned into the line and degrades transmission characteristics.
To avoid this reflection, proposals have been made, in particular in the article "Wideband gain flattened erbium fiber amplifier using a photosensitive fiber blazed grating" by R. Kashyap, R. Wyatt, and R.J. Campbell, published in Electronics Letters of Jan. 21, 1993, Vol. 29, No. 2, pp. 154 to 156, to incline the fringes representative of index modulation zones. This can be done by causing interference between two beams from a frequency-doubled argon laser source providing a wavelength of 244 nm, and by inclining the normal to the section that serves as a filter relative to the bisector of the two exposure beams. It is also possible to use a phase mask that generates mainly two diffraction orders: +1 and -1; together with a zero order that is very weak. In the above-mentioned article, the inclination is eight degrees, for example. The advantage of the inclination is to eliminate reflection. The effect of the inclination is to couple the fundamental mode propagating forwards with the radiative modes propagating in the opposite direction. These radiative modes are absorbed very quickly by the cladding, and they are referred to as "cladding modes". The spectral envelope of the set of frequency components in these various cladding modes can then be used as a characteristic of a filter for compensating the gain of optical amplifiers.
The drawback presented by that technique lies in the selectivity of the filter. When using standard telecommunications fibers, it is not possible with such a Bragg grating filter having inclined index changes to obtain a filter bandwidth of less than 20 nm, for example. In theory it is possible to act on the diameter of the core so as to reduce the bandwidth of the filter. The filter is thus more selective if the diameter of its core is larger, e.g. 9 .mu.m instead of 3 .mu.m. However this increase in diameter is limited. In addition, amongst other drawbacks, it has the drawback of requiring matching sections to be made between fiber with a large diameter core and fiber with a standard diameter core (already about 9 .mu.m). Such matching is difficult to achieve.
Depending on the desired outcome, the attenuation of cladding modes is improved but the length of the grating can no longer be used to narrow the bandwidth of the filter. In practice, the smaller the angle, the more the filter can be selective, but simultaneously the greater the amount of residual emission by reflection of the type that occurs with right fringes. In contrast, the more the angle slopes, the smaller the effect of the reflection phenomenon but the wider the band of the filter, i.e. the filter is less selective. In any event, the compromise that is obtained is not satisfactory and attempts are being made to improve it.
A second problem with that type of filter lies in a secondary filter peak or "rebound" in a lower frequency band close to the working band where filtering is desired. This rebound is due to the above-mentioned residual reflection in fundamental mode. Initially the rebound is not troublesome since known optical amplifiers are of limited bandwidth and the filter rebound lies outside it. But even then, the rebound must remain small. However in other applications, in particular in land applications, the filter is used selectively to attenuate different components within the working band. The spectral position of the rebound therefore also lies within the working band. In these other applications, the filtering rebound is therefore harmful.
Thirdly, as has been mentioned before, the attenuation spectrum is in fact an envelope of attenuations of different spectrum components. This means that within the envelope some spectral components are filtered effectively while others are filtered less well, or even not at all. This is due to the cladding modes being discrete. Under such conditions, the filtering envelope corresponds to a superposition of discrete and relatively narrow band filters that are spaced apart from one another by frequency gaps where filtering is not performed. Such a filter therefore cannot be used to equalize the gain of optical amplifiers correctly.